A liquid crystal display is mainly consists of a liquid crystal display panel and a backlight unit. The liquid crystal display panel includes a color filter substrate, a thin film transistor substrate, and a liquid crystal layer interposed between the thin film transistor substrate and the color filter substrate. Since the liquid crystal display panel itself is a non-emissive device, the backlight unit is required to provide light for achieving the display function; that is, the backlight unit is employed to provide sufficient brightness and uniform light source, thereby allowing the liquid crystal display to display images normally.
Presently, the development of the light-emitting diode (LED) light source has a breakthrough influence on the liquid crystal display products. The significant improvement on the luminous efficacy of the LED allows it to have half the efficiency of a cold cathode fluorescent lamp (CCFL). Further, LED is a low-power spontaneous light source, usually used as auxiliary light source for power efficiency products. Hence, various studies had equipped the backlight unit of liquid crystal display with LEDs as a light source thereof.
Generally, when using LEDs as a backlight light source, a plurality of LEDs is electrically connected to each other in series. Therefore, in order to drive the LEDs, a higher direct voltage (DC voltage) is required. As a result, a DC-DC boost converter is provided in the driving system of the backlight unit to drive the LEDs.
FIG. 1 illustrates a schematic diagram of the DC-DC boost converter. The DC-DC boost converter 100 includes a DC power source 110, an inductor 120, an output capacitor 150, a diode 130, and a bipolar junction transistor (BJT) 140.
The DC power source 110 includes a negative electrode terminal and a positive electrode terminal connected to one end of the inductor 120. The other end of the inductor 120 is connected to the collector of the BJT 140 and the anode of the diode 130. The cathode of the diode 130 is grounded via the output capacitor 150. The emitter of the BJT 140 is grounded, and the base of the BJT 140 is electrically connected to a switch circuit 160. The switch circuit 160 is used to control the on and off of the BJT 140. Here, the voltage of the DC power source 110 equals to the input voltage of the DC-DC boost converter 100.
In the following description, referring to FIG. 1, the action of the DC-DC boost converter 100 will be explained. As shown in FIG. 1, it is assumed that when the switch circuit 160 provides the BJT 140 with a voltage signal of low electric potential, the BJT 140 is turned off, and after a sufficient period of time, all the components are in ideal state and the voltage of the two ends of the output capacitor 150 equals to the input voltage.
Next, charging and discharging process of the boost converter will be described. During the charging process, the switch circuit 160 provides the base of the BJT 140 with a voltage signal of high electric potential, and the BJT 140 is turned on. At this time, electrical current from the DC power source 110 flows through the inductor. The diode 130 prevents the capacitor from discharging to the ground. Since the DC power source 110 inputs direct current, the electrical current within the inductor 120 increases linearly by a constant ratio, wherein the ratio is related to the size of the inductor 120. With the current within the inductor 120 increases, the energy stored in the inductor 120 also grows.
During the discharging process, the switch circuit 160 provides the base of the BJT 140 with a voltage signal of low electric potential, and the BJT 140 is turned off. At this time, owing to the characteristics of the inductor 120, the current flowing through thereof will go slowly from the initial current value stored during the discharging process to zero, instead of becoming zero immediately. The original circuit is broken; hence, the current of the inductor 120 may only discharge through the output capacitor 150. That is, when the inductor 120 begins to charge the output capacitor 150, the voltage of the two ends of the output capacitor 150 will rise. The voltage of the two ends of the output capacitor 150 equals to the output voltage of the DC-DC boost converter 100.
In practice, during the charging process, the current flowing through the inductor 120 has a constant maximum value. Thus, if the difference between the input voltage and the output voltage is too big, for instance, the input voltage being 24V and the output voltage being 300V, then it requires multiple charging and discharging processes to achieve desired output voltage. Therefore, the more often the DC-DC boost converter 100 charges and discharges, the faster the output voltage rises.
However, when the output voltage reaches the desired voltage value, the loading (not shown) of the DC-DC boost converter 100 starts to work normally, which means that the loading begins the power consumption thereof, decreasing the voltage of the two ends of the output capacitor 150. In order to maintain a constant output voltage, the DC-DC boost converter 100 has to continue the charging and discharging process.
An oscillator is generally used in the switch circuit of the backlight unit for outputting a constant frequency in order to control the on and off of the bipolar junction transistor, and thereby maintain the stability of the output voltage. Yet, when turning on the liquid crystal display, a certain amount of time is required to allow the voltage of the two ends of the output capacitor to reach the voltage value enough for driving the loading. The loading herein refers to the light emitting diodes employed as the light source of the backlight unit. Due to the characteristics of the light emitting diode, a small amount of electrical current will flow through the light emitting diode when the output voltage rises to a certain value even though the desired voltage value has not yet reached, causing the light emitting diodes to glimmer a faint light. At this point, because the voltage rising time takes long enough, human eyes may see the light emitting diodes lighting up slowly, and perceive that the light emitting diodes are flickering.
Hence, a method for driving backlight unit of liquid crystal display and the system thereof that could prevent human eyes from perceiving the flickering of the LEDs is required in order to overcome the foregoing deficiencies.